(a) Technical Field
The present invention relates to a metallic bipolar plate for a fuel cell and a method for forming a surface layer of the same. More particularly, the present invention relates to a metallic bipolar plate for a fuel cell and a method for forming a surface layer of the same, in which the surface layer is preferably formed in such a manner that a plasma nitridation process is performed on the surface of a stainless steel base material to form a nitrogen implanted layer, and oxidation and reduction processes are performed on the surface of the nitrogen implanted layer to form a Fe3O4 surface oxidation layer thereon, and thus suitably improving conductivity and corrosion resistance.
(b) Background Art
A fuel cell is an electricity generation system that does not convert chemical energy of fuel into heat by combustion, but electrochemically converts the chemical energy of fuel directly into electrical energy in a fuel cell stack. Such a fuel cell can be used in the supply of electric power for small-sized electrical/electronic devices such as portable devices, as well as in the supply of electric power for industry, home, and vehicle.
At present, the most attractive fuel cell for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC) having the highest power density among the fuel cells. The polymer electrolyte membrane fuel cell has a fast start-up time and a fast reaction time for power conversion due to its low operation temperature.
The polymer electrolyte membrane fuel cell typically comprises: a membrane electrode assembly (MEA) including a polymer electrolyte membrane for transporting hydrogen ions and an electrode catalyst layer, in which an electrochemical reaction takes place, disposed on both sides of the polymer electrolyte membrane; a gas diffusion layer (GDL) for uniformly diffusing reactant gases and transmitting generated electricity; a gasket and a sealing member for maintaining airtightness of the reactant gases and coolant and providing an appropriate bonding pressure; and a bipolar plate for transferring the reactant gases and coolant.
In the fuel cell having the above-described configuration, hydrogen as a fuel and oxygen (air) as an oxidizing agent are supplied to an anode and a cathode through flow fields of the bipolar plate, respectively. The hydrogen is supplied to the anode (also called a “fuel electrode”, “hydrogen electrode”, and “oxidation electrode”) and the oxygen (air) is supplied to the cathode (also called an “air electrode”, “oxygen electrode”, and “reduction electrode”).
The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by catalyst of the electrode catalyst layer provided on both sides of the electrolyte membrane. At this time, only the hydrogen ions are transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and at the same time the electrons are transmitted to the anode through the GDL and the bipolar plate, which are conductors.
At the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate meet the oxygen in the air supplied to the cathode by an air supplier to cause a reaction that produces water.
Due to the movement of hydrogen ions caused at this time, the flow of electrons through an external conducting wire occurs, thus generating a current.
The electrode reactions in the polymer electrolyte membrane fuel cell can be represented by the following formulas:Reaction at the anode: 2H2→4H++4e−Reaction at the cathode: O2+4H++4e−→2H2OOverall reaction: 2H2+O2→2H2O+electrical energy+heat energy
The above-described bipolar plate in the fuel cell is a core component together with the MEA and performs various functions such as structural support of the MEA and GDL, collection and transmission of the generated current, transport of the reactant gases, transport and removal of the reaction by-products, and transport of the coolant for removing the reaction heat. Accordingly, the bipolar plate should have characteristics such as excellent corrosion resistance, airtightness, and chemical stability.
Conventional bipolar plates are formed of a graphite material or a composite graphite material, in which resin and graphite are mixed, having excellent electrical conductivity and chemical stability. However, the graphite bipolar plate has drawbacks in that it has mechanical strength and airtightness lower than those of a metallic bipolar plate and has high manufacturing cost and low productivity since the manufacturing process is performed manually without the use of a machine due to its fragility.
Accordingly, extensive research aimed at substituting the graphite bipolar plate by the metallic bipolar plate has been conducted.
Surface treatment methods of a stainless steel bipolar plate as the metallic bipolar plate for the fuel cell are broadly classified into two methods. According to the first method, carbide or nitride is coated on the surface of the stainless steel bipolar plate by physical vapor deposition (PVD), that is, a coating layer of chromium nitride (CrN) or titanium nitride (TiN) is formed on the surface thereof. According to the second method, the surface of the stainless steel bipolar plate is modified by nitriding or carburizing. For the surface modification, extensive research aimed at improving surface characteristics by forming a nitride layer by plasma nitridation at a temperature below 600° C. has been conducted.
The CrN coating layer formed by the physical vapor deposition has excellent corrosion resistance; however, it has drawbacks in that the contact resistance is relatively high, and the cost is high. Especially, since the PVD coating of CrN, TiN, etc. can provide a coating layer of good quality, a desired performance of the fuel cell bipolar plate can be obtained; however, it requires a high vacuum process and it has limitations in terms of manufacturing cost and mass productivity.
Meanwhile, the surface modification method such as nitriding has advantages in that the manufacturing cost is low and the mass productivity is excellent; however, it deteriorates the characteristics of the base material, and thus the corrosion resistance is reduced. In case of the surface nitride layer formed by the plasma nitridation has excellent cost competitiveness; however, since the surface nitride layer forms a nitride by combining with chromium (Cr) of the base material, it consumes chromium of the base material to form a chromium depletion layer having numerous pores on the surface thereof, and thus the corrosion resistance of the surface layer is reduced. If the chromium depletion layer is formed on the stainless steel surface layer as chromium of the base material is nitrided, the surface of the steel is oxidized and corroded. Moreover, if a thin oxide is formed on the surface layer, the corrosion resistance is improved; however, the contact resistance of the surface is excessively increased, and thus the bipolar plate no longer functions.
Accordingly, in order to apply the stainless steel to the fuel cell bipolar plate, it is necessary to provide a surface structure and a surface treatment process, which can prevent the nitration of chromium and the formation of an oxidation layer in a low temperature process and improve the corrosion resistance by minimizing surface defects.
Japanese Patent Publication No. 2000-353531 discloses a technique for forming a chromium nitride such as CrN, Cr2N, CrN2 and Cr(N3)3 by coating chromium on the surface of a base material and performing a nitridation process. This technique requires a reduction in temperature and time of the nitridation process to improve the mass productivity and reduce the manufacturing cost. In forming the chromium nitride layer as a protective layer, if the temperature and time of the nitridation process are reduced, it is difficult to ensure a desired corrosion resistance.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.